This invention relates to a probe for measuring workpieces. A known such probe (U.K. Pat. No. 1,237,813) has an elongate stylus supported by a fixed member and a free end for engagement with the workpiece to be measured. The probe is intended to be mounted in a machine which can move the probe relative to the workpiece and which determines the instantaneous position of the probe in three dimensions relative to a datum position. To measure a given surface the machine moves the probe toward that surface and then halts the probe in a predetermined position, relative to the datum. Towards the end of the movement the free end of the stylus engages said surface and during the remainder of the movement the stylus becomes displaced by a small amount relative to the fixed member. This displacement is measured by a transducer connected between the stylus and the fixed member. If, when the probe is halted, the transducer output is zero, then the position of the surface is correct, i.e. conforms to the predetermined position. Any departure of the transducer output from zero indicates the extent of an incorrectness in the position of the surface.
To satisfy the requirement for three-dimensional operation, the stylus is supported on the fixed member for universal pivotal motion and for longitudinal motion. The pivotal motion occurs when the free end of the stylus is acted on in a direction transverse to the length of the stylus while the longitudinal motion occurs when said free end is acted on in the direction of the length of the stylus. Such an arrangement is advantageous from the point of view of mechanical construction.
Further, the known probe has a single transducer to indicate the extent of said displacement. This is desirable because inter alia it avoids the need to calibrate two or more transducers (which might be used for such probes) relative to one another. However, there is the difficulty that, due to the different geometries of the pivotal and the longitudinal motions, displacement of the free end of the stylus by a given distance results in different outputs of the transducer according to whether the motion of the stylus was pivotal or longitudinal. This makes it impossible to use a single calibration, i.e. a single zero setting, of the transducer for transverse and longitudinal displacement. The known probe copes with this difficulty by providing a cam and a slide between the stylus and the transducer but this is unsuitable for high accuracy applications because of the well-understood difficulty of friction and of play in such mechanical devices. It is an object of this invention to reduce or overcome this difficulty.
Further, in the known probe, the output of the single transducer does not discriminate between different axes of the orthogonal coordinate system. While this is not necessarily a difficulty if the surface to be measured is perpendicular to the direction in which the measurement is made, a difficulty arises if the measurement has to be taken at a point on a surface oblique to that direction. In such a case the stylus tends to slide along the oblique surface so that the displacement in the required direction of measurement is lost. It is a further, optional, object of this invention to overcome the latter difficulty also.